Means for receiving high frequency signals



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I MEANS FOR RECEIVING HIGH FREQUENCY SIGNALS AMPLIFIER 6 TUNING v TUNING DISCRIMINATOR DISCRIMINATOR FILTER FILTER INVENTOR. 914.5122 7128140778110 BY ATTORNEY.

y 2, 0- G. GUANELLA MEANS FOR RECEIVING HIGH FREQUENCY SIGNALS Filed May 17. 1938 4 Sheets-Sheet 4 INVENTOR,

ATTORNEY.

l \l gu-sfavejuanella Patented July 2,1940

MEANS FOR RECEIVING SIGNAL Zurich, Switzerland, assignor Gustave Guanella, to Radio Patents gIGB FREQUENCY a corporation ofNew York Corporation, New, York, N. I,

In Switzerland July 10, 1937 Application May 17, 1938, Serial No. 208,366

8 Claims. (01. 250-) The present invention relates .to means for receiving high frequency signals, more particularly to the reception of modulated signalshaving a carrier or unmodulated component subject to rapid fluctuations of both. regular and irregular character and due to various causes and origins.

As is well known, in modulating the. amplitude of a high frequency carrier wave it is impossible in most cases to avoid a simultaneous frequency or phase modulation manifested by fluctuations of the carrier frequency in the rhythm of the low frequency or modulating signal to be transmitted. These fluctuations in general amount to only a small fraction of the average carrier frequency. However, the range of the resultant frequency band transmitted may be considerably increased beyond the range determined by the low fre-' quency modulation. As a result, a selective receiver having a considerably extended response or receiving band width is required for the reception of such oscillations, since in case of a limited response characteristic of the receiver an undesired, non-linear conversion of the frequency modulation into an amplitude modulation will occur making reception in most cases impossible.

Aside from this fact, it is .generally required in a receiver employing band-pass filters with subsequentv rectification that the carrier frequency at any time coincides with the medium or central response frequency of the filter or receiving circuits to avoid asymmetrical phase rotation in the side. bands liable to lead to nonlinear distortion during rectification. For these reasons, great difficulties are experienced in receivingamplitude modulated oscillations subject to carrier frequency fluctuations.

In case of receiving short waves, selective circuits can be easily constructed having the necessary extended bandwidth comprising both the modulation side bands and the carrier frequency fluctuation range since in this case the resultant band is still small compared with the carrier frequencies employed. However, it is understood that due to the extended receiving band width or resonance characteristics disturbances and interference of various origins having frequency 7 components falling within the extended transmitting range of the selective circuits are received in an undesirable manner together with the signal frequencies.

When employing superheterodyne reception, additional difiiculties are encountered in case of a fluctuating carrier frequency due to the ex- 55 tended frequency range and in view of the fact that the frequency range is no longer small compared with the intermediate frequency employed in receivers of this type. For this reason, the employment of superheterodyne reception for short waves has been difficult in most cases and 5 entirely unfeasible and impractical in the case of ultra short waves. Thus, the special advantages of frequency transformation and subsequent amplification employed in the superheterodyne type a of reception cannot be utilized for the reception of ultra short waves,the straight amplification of which, as is known, is also accompanied by substantial difficulties.

Accordingly, it is an object of the present invention to substantially overcome the difficulties experienced in' receiving amplitude or otherwise modulated carrier signals the carrier frequency of which is subject to rapid fluctuations of the order of the modulating frequencies to betransmitted due to additional frequency or phase 29 modulation or to other causes.

Another object is to provide a receiving system for modulated carrier signals subject to rapid fluctuations of the carrier frequency using selective receiving means having a band width or response whichis equal to or only slightly exceeds the range of the modulation side bands of the signals to be received.

Another object is to provide a system and a method for receiving a high frequency carrier wave modulated according to a signal wave to be transmitted and being subject to periodic fluctuations of the carrier frequency according to the instantaneous values of said signal wave, enabling the employment of sharply selective circuit means 5 having frequency, response characteristics not substantially exceeding the side band or modulation range occasioned by the non-fluctuating carrier wave modulated in accordance with said signal wave. g

A more specific object is to provide a system and a method for receiving a high frequency carrier wave having an amplitude modulated according to a signal wave to be transmitted and being subject to periodic fluctuations of. the carrier frequency according to the instantaneous values of said signal wave, enabling the employment of a sharply tuned band-pass receiver having a response characteristic (band width) not substantially exceeding the side band range occasioned by the amplitude modulation of the constant non-fluctuating carrier in accordance with said signal wave.

A further object is the provision of a receivcarrier wave subject to fluctuations of the carrier bodied therein to improve frequency due to additional frequency and/or phasemodulation, are substantially overcome by the novel method and system proposed by the invention. By the new method it is possible to employ selective circuits or networks having a band width or response characteristic which comprise or exceed only slightly the frequency range occasioned by the low frequency modulation or signal side band range and by which a superheterodyne reception is enabled for'the shortest wave lengths at present used in practice.

As is known, an amplitude, frequency or phase modulated carrier wave may be resolved into a greater or lesser number of components of different frequency corresponding'to the low frequency or signal wave which in their entirety may be substituted by a single frequency of variable amplitude or frequency.

Further objects and novel aspects of the invention will become more apparent from the following detailed description of practical embodiments thereof taken with reference to the accompanying drawings forming part of this specification and wherein:

Figures 1 to 4 represent diagrams explanatory of the function and operation of the invention,

Figure 5 shows by way of example a discriminating circuit suitable for use in a receiving system of the type according to the invention,

Figure 6 shows in block diagram form an inventive receiving system embodied in a. straight or radio frequency (RF) amplifier,

Figure '7 shows a modified receiving system embodied in a superheterodyne'receiver,

Figures 8 and 9 show partial circuit arrangements to be employed in the previous circuits to improve the operation thereof Figures 10 and 11 show modified receiving syste'ms of the type shown in Figures 6 and 7, respectively, with features of improvement emthe operation,

Figure 12 shows a receiving system according to the invention employed in a multiple superheterodyne receiver,

Figure 13 is a complete circuit diagram for a superheterodyne receiver suited for short wave and ultra-short wave reception constructed according to the invention.

Like reference characters indicate like parts throughout the different views of the drawings.

With the above objects in view, the present :invention contemplates the generation of a tuning control potentialvvarying according to the amount of instantaneous detuning of a receiver relative to the instantaneous frequency of a received carrier frequency wave. This tuning control potential serves to control the tuning adjustment of the receiver in such a manner as to maintain the detuning at each instant within a sufficiently small limit independently of rapid fluctuations of the carrier frequency of the received signals such as fluctuations due to additional frequency modulation in case of an amplitude modulated signal according to a low frequency or modulating signal wave being transmitted which may be within the audible range.

Exact analysis has shown that the instantaneous frequency of an oscillatory circuit having variable parameters such as variable inductance or capacitance elements actually corresponds to the resonance frequency which may be computed equivalent circuit with fixed parameters. However, in case of periodically fluctuating parameters corresponding amplitude variations occur in general which in the present case are of secondary importance in view of the fact that the frequency fluctuations are slight in comparison to the average value of'the carrier or tuning frequency. In the practically rare cases where this is not the case, the above amplitude fluctuations may be eliminated by a rapidly acting control arrangement of the type of the known automatic volume control (AVC) systems commonly used in radio receivers. Such amplitude fluctuations may also be avoided by simultaneously varying both the inductance and capacitance of the oscillatory circuit in equal amounts.

Investigation has further shown that in case of periodically varying tuning parameters, certain natural frequencies may build up excessive amplitudes and cause oscillations within certain unstable operating regions. This phenomena occurs only if the frequency of the tuning variations differs substantially from the average tuning (carrier) frequency of the oscillatory circuit and they may be eliminated by providing sufficient damping in the circuits.

Since the tuning of the receiver according to the present invention is regulated so as to constantly follow the instantaneous frequency of the received carrier wave, it will be referred to hereafter as synchronized tuning for the purpose of this specification. In this respect, that is in the instantaneous control of the tuning which follows the most rapid variations of the received frequency, the invention differs funda-' result of a relatively slow carrier frequency change known as drift. In accordance with this special purpose and function a rapid action in the known arrangements for automatic tuning is not desirable nor required, the usual time constants being of the order of one second. In contrast thereto, time constants of the order of one thousandth of a second are required to eliminate the effect of rapid fluctuations at a rate of the order of the component frequencies of the modu-' lating signal wave being transmitted such as in case of additional frequency modulation to secure a synchronized tuning in accordance with the objects and function to be obtained by the present invention.

Referring to the diagrams according to Figures 1 m3 of the drawings wherein are plotted frequencies as a function of time, ft represents the carrier frequency and f1 and is the upper and lower modulation side frequencies of an amplitude modulated high frequency carrier wave. In the case of Figure l, the carrier frequency ft is assumed to be constant, that is there is no additional frequency modulation or fluctuation and accordingly the side band frequencies f1 and also have a constant value. Signals of this type may be received by means of a selective circuit having a response characteristic A with a band.

quency or side band range of the signals as shown by the diagram in Figure 1 representing signal response a as a function of the frequency 1. An interfering signal comprising a frequency range indicated by the arrow 2' and occurring at a time t1 is completely eliminated by the action of the selective circuit as' is readily understood.

If, on the other hand, the carrier frequency for one reason or another (such as due to an additional frequency modulation as explained hereinbefore) is subject to rapid fluctuations, conditions are changed as represented in Figure 2. In the latter, it is assumed that the carrier in and with it the side band frequencies f1 and f2 fluctuate rapidly according to a sinusoidal variation. A reception of signals of this type with normal means is possible only if the resonance characteristic A of the receiver has a band width b encompassing both the modulation frequency range and the range of the carrier fluctuations such as shown in the illustration. In this case, the interfering frequencies 2' fall within the response range of the receiver and accordingly will be received together with the desired signals.

Referring to Figure 3 which illustrates the operation of a receiver proposed by the invention,

the tuning of the receiver is varied in synchronism with the carrier frequency it such as indicated by the band filter curves A1, A2, A3, corresponding to instants t1, t2 and ta, respectively, during the fluctuating cycle. In this case, the disturbing frequencies 1' are no longer received together with the signal frequency due to the instantaneous location of the resonance characteristic A1 outside the disturbing signal range 2' at the instant t1.

For the generation of a tuning responsive control potential, apparatus is required having a characteristic as shown by the diagram according to Figure 4 wherein it represents the tuning responsive potential as a function of signal frequency f. There is thus required a tuning control potential u varying in dependence upon the instantaneous differential A) between the received instantaneous frequency f and the natural or resonant frequency F of the receiver which control potential at each instant varies both according to the sense and amount of detuning A). The received high frequency j which in the example illustrated is somewhat above the tuning frequency F will cause the generation of a corresponding positive control potential adapted to effect the necessary instantaneous tuning control within the required short time period. Ifthe received frequency f is below the tuning frequency F, a similar control potential but of opposite polarity will be generated. If the received signals comprise the side bands s1, 82 due to modulation in accordance with a signal wave, the same control potential should be produced, that is the control potential should not be dependent upon the instantaneous condition of the amplitude modulation. This is the case if the characteristic according to Figure 4 is substantially a straight line within the operating range comprised by the carrier and side frequencies. Thus, an approximate linear portion of the curve is required bl.- tween the points c and d, i. e. within a range comprising the greatest deviations to be expected between the received frequencies and the instantaneous tuning frequency of the receiving circuit. Beyond these points, a rapid descent of the curve to thezero line is desirable in order to prevent the generation of a control potential by described hereinafter.

disturbing frequencies liable to interfere with the tuning control operation.

For the generation of a control potential of the type according to Figure 4 a system of known type used for automatic frequency control (AFC) for radio receivers properly modified may be employed for the purpose of. this invention.

An arrangement of this type is shown by way of example in Figure 5. According to the latter, the fluctuating high frequency potential is im pressed via terminals 111 and 11 upon a primary resonant circuit comprising a capacity l shunted by an inductance II. This circuit is inductively coupled with a secondary resonant circuit comprising a capacity i2 and inductance 13 thereby forming what is generally known as a band-pass filter or network. In a circuit of this type if tuned to the average or carrier frequency, the voltage across the secondary has amutual phase relative to the voltage across the primary which varies between 0 and 180 as thefrequency of the impressed potential varies relative to the natural or tuning frequency of the network. By connecting the primary circuit to the electrical center of the secondary such as to the center tap of the inductance l3 as shown, the sum and difference of both primary and secondary potentials are formed which sum and difference poten-- tials are separately rectified such as by the aid of a double diode rectifier l6, thereby producing a tuning control potential at terminals o-p varying in dependenceupon the sense and amount of detuning of the network relative to the impressed signal frequency. The condensers l4 and it; are provided to block the tuning control potential from the resonant circuits while numerals I8 and 19 represent leakage resistances, 20 and 2| are smoothing resistances, and 22 is a smoothing condenser for eliminating the high frequency ripples or fluctuations in the control potential. In order to ensure a rapid and instantaneous variation of the control potential corresponding to rapid frequency fluctuations, the time constant of the combination l8M-l9|5 and of (20+2l)-22 should have, a sufliciently low value. In addition to the tuning control potentiaL'a further potential varying according to the carrier amplitude or signal strength may be derived from the junction point of a pair of resistances 23 and 24 placed across the output circuit and one side of the input circuit, terminals. q--r. This amplitude responsive control potential may serve for automatic volume control or for a special amplitude control As is understood, any other known frequency discriminator circuit arrangement having properly designed circuit constants to suit the special requirements of the in vention may be employed for the purpose described.

In order to control a parameter or element determining the tuning of an oscillatory circuit such as an inductance or capacitance element in dependence upon thetuning control potential various methods suggested in connection with arrangements for automatic frequency control (AFC). in radio receivers may be employed in practicing the present invention. Thus, an electron valve may be employed as a variable reactance element forming part of an oscillatory or resonant circuit whereby the reactance of such valve is controlled by adjusting a suitable biasing potential. Alternatively, an iron cored inductance coil forming a tuning element in an oscillatory or resonant circuit may be provided with an auxiliary pre-magnetization winding energized by the tuning control potential.

Referring to Figure 6, there is shown in block diagram form a basic receiving circuit of the type according to the invention. Numeral 28 represents a tuned radio frequency (RF) amplifier upon which is impressed an input potential wave e1 having a fluctuating carrier frequency and amplified to a value e2. The amplified potential 62 is applied to a tuning discriminator 29 which may be of the type according to Figure 5 or of any other suitable construction known. In this manner, a tuning control potential ur is generated by the discriminator which varies both in polarity and magnitude according to the sense and in proportion to the amount of detuning of the receiver relative to the frequency of the impressed signal potential e2. If desired, this control potential 1L1 may be smoothened by means of a filter 30 which latter should be permeable for the low (fluctuating) frequency variations of the control potential. The thus obtained control potential us which fluctuates in accordance with the modulating signal wave or any other carrier frequency variation of e1 or ez is utilized to control the tuned circuits in the receiver radio frequency amplifier 21 which amplifier has a narrow band width characteristic varied according to the control potential as and producing an output signal e3. The tuned circuits of the RF amplifier 28, tuning discriminator 29 and receiver amplifier 21 are preferably ganged so as to simultaneously tune these circuits. The control potential uz may also if desired be used to control the tuned circuits in both the amplifier 28 and the discriminator 29 in such a manner that the amount of detuning relative to the instantaneous carrier frequency and in turn the regulating potential uz are maintained within a predetermined minimum range. The potential es may be rectified and is substantially free from interfering signals whose frequency is outside the instantaneous band-pass or response range of the tuned amplifier 21. In the case of straight high frequency'amplification, it has been found desirable to control the tuning of the RF amplifier 28 and the discriminator 29 simultaneously with the tuning of the amplifier 21 as shown and described. In the latter case it is also possible to omit the receiver amplifier 21 and to use the output signal at of the amplifier 28 for detection and translation of the low frequency signals to be received.

Referring to Figure 7, there is shown a similar receiving system embodied in a superheterodyne receiver. In this modification the input potential e0 having a fluctuating carrier frequency is impressed upon a mixer 3| of any known type-and construction and combined therein with a potential of an auxiliary frequency at produced by a local oscillator 32 in'a manner well known. The difference (or sum) frequency e1 obtained in this manner is applied to an intermediate frequency (I.F.) amplifier 33 and the amplified intermediate frequency signal 6:: impressed upon the tuning discriminator 34. The latter is fixedly tuned to the invariant frequency of the selective or band filter circuits of the I.F. amplifier 33. Thus, the tuning and control potential u1 produced by the discriminator 34 varies in accordance with the instantaneous deviation of the intermediate carrier frequencyfrom the fixed intermediate tuning frequency of the amplifier 33.

This potential (in) may be smoothened through a filter 35 and the filtered potential (us) is applied to the oscillator 32 to regulate the oscillator frequency in such a manner that the deviation of intermediate frequency e1 or 62 obtained by the action of the mixer 3| always remains within a desired predetermined limit range. The intermediate frequency signals may be further amplified such as by a separate amplifier 36 and rectified to operate an output or translating device such as a loud-speaker in a manner well known. If required, any existing radio frequency (RF) pre-selector circuits may be controlled by the same tuning control potential, otherwise the pre-selecting circuits should be designed with a sufficiently wide response band width.

- In the design of the control and discriminator circuits, certain requirements have to be complied with in an effort to avoid self-excitation and hunting effects within the regulating system and to ensure a sufficiently rapid and instantaneous regulation for the fastest fluctuations to be expected of the received carrier frequency.

This can also be expressed by the requirement that the phase difference between all periodic frequency variations to be expected and the tuning variations caused by the corresponding variations of the tuning control potential should always remain smaller than This behavior and the tendency to produce hunting in case of frequency changes can be best understood by considering a single sudden frequency change. In this connection, there is to be considered in the first place the so-called regulating factor R by which is understood the quotient between a predetermined deviation of the received carrier frequency and the tuning variation caused by a tuning control potential produced as a result of such deviation. Furthermore, there is to be considered the so-called transit period 151 and the transition time t: within the regulating cycle. By ii there is understood the time which has elapsed after a sudden frequency change until the initiation of the tuning regulation and by 132 there is understood the duration of the regulating action corresponding to a definite sudden or permanent detuning in the frequency control arrangements 29 or 34 according to Figures 6 and '7, respectively, It is advisable to assume a linear relationship of the tuning variation as a function of time which coincides with the steep portion of the actual characteristic curve, whereby t2 is defined as the time period within this linear increase to the instant when a constant final value is reached. With these assumptions made the condition for the stability may be represented with sufiicient approximation by the following mathematical expression (l-it) The transit time h is determined primarily by the time constant of the amplifier for a definite tial extent upon the band lective circuits in the amplifiers of a fast acting high frequency signal and depends to a substanwidth or response characteristic of the selective circuits (band-pass filters). This transit time is characteristic for the most rapid frequencyvariations for which a tuning control may still be expected. In the interest of an eflective tuning regulation with rapid frequency fluctuations it is further desirable that the transition time t: shouldbe as short as possible, which transition time is determined mainly by the time constant of' the discriminator 23 or 34 and the smoothing filter 30 .or 35, Figures 6 and 7, respectively. v

Since in most cases the band width of the se- 28 and, Figures 6 and 7, respectively, should be considerably larger than the minimum band width determined by the low frequency modulation in order to obtain small transit periods as pointed out above. it is advisable to provide special amplifiers designed with a narrow band width for the normal demodulation and translation of the high frequency or intermediate frequency signals, in order to prevent reception or disturbing frequencies to the greatest possible extent. These special amplifiers are shown at 21 and 36 in Figures 6 and '7, respectively, connected ahead ofthe RF or IF amplifiers, respectively, and supply an output signal ea to be demodulated and further amplified for operating an output or translating device. The amplifier 36 differs fier 21 however in that the narrow band width is constant at one frequency therefore requiring no tuning control whereas the amplifier 21 being of the RF tuned type having a narrow band width characteristic necessitates the tuning frequency to vary according to the tuning control potential so as to be receptive to the varying received carrier frequency.

In the known arrangement for automatic frequency control (AFC) used in radio receivers, the tuning control potential is smoothened to a considerable extent, the time constants required being of the order of one second. In the case tuning control according to the invention which instantaneously follows the rapid frequency changes of a received carrier frequency wave, time constants which are by several thousandths smaller are required in general, i. e. the transition period should be of the order of the period of the fastest frequency fluctuation to be expected.

The following is to be considered, according to a further feature of the invention. In case of large frequency variations of a received carrier signal it may happen that theregulating action is interrupted or disturbed by a momentary cessation of this frequency or by a short impulse caused by a disturbing frequency. In this case, the automatic receiver tuning may cease to function and remain at a fixed value whereby reception will in general be interrupted until a wave with a similar instantaneous frequency is received and synchronized tuning restored. This may be caused by a separate transmitter of higher or' lower carrier frequency. This instability or jumping upon other frequencies is eliminated ligible.

a disturbance of erodyne within a short period and the interruption is scarcely noticeable. This can be accomplished by a suitable design of the transmission circuits for the regulating potential such as the filters 30 and II. Figures 6 and 7, respectively. Suitable transmission networks for this purpose are shown in Figures 8 and 9.

According to Figure 8, the input terminals 0- are connected to the discriminator circuits 29 or 34, Figures 6 and 7, respectively, and a tuning control potential appliedfrom the output terminals &.t to the resonant circuit such as the tank circuit in the oscillator of a superhetreceiver to be controlled. The condenser 3! serves to transmit the rapidly varying tuning control potential corresponding to a frequency or phase modulation of the received oscillations without appreciable attenuation and time lag due to the fact that the by-passing of these frequencies through the large resistance 39 is neg- I'he condenser 31 having a capacitance substantially higher than the capacitance of the condenser 38, on the other hand, is charged through resistance 39 and'a further still higher resistance 0 with a potential corresponding to the average value of the tuning control potential. It the regulating action has been interrupted by any kind, the instantaneous charge of the small capacity 38 is discharged in a short period through the resistance 40 in such a manner that the potential of the condenser 31 appears at the terminals st, that is a control potential corresponding to the average frequency of the oscillations being received. In this manner, the synchronization with the received high frequency oscillations is restored within a short period. Thus, by the action of the resistance Ml and condenser 31 a highly steadied or smoothened control potential is produced for effecting a slow acting automatic frequency control while through the action of the condenser 38 there is produced arapidly varying control potential for effecting synchronized tuning in accordance with rapid tuning variations caused by additional frequency or phase modulation or to other causes. I

Referring to Figure 9 there is shown an alternative circuit for producing both slow andrapid tuning control potentials. In this case the control potentials for the slow automatic frequency control are passed through the circuits til, 38

according to a further feature of the invention by an arrangement whereby the tuning of the receiver after each interruption is automatically returned to the proper value .of the high freand M, 383 while the rapid control potentials for automatic synchronization are passed through the circuits 31, 39 and 31, 39. It may be advantageous to provide suitable limiting devices in the transmission path for the synchronizing potentials in order to suppress excessively high frequencies in the control potential which may be caused by disturbing phenomena and which are not desired in the tuning regulation. Furthermore, the slow and rapid regulations may be separated in the case of superheterodyne reception employing a double heterodyning method whereby one local oscillator is utilized to eifect the slow acting (AFC) control while the other oscillator serves for efl'ecting the fast acting con- .trol or synchronization.

Difllculties may be experienced in carrying out a synchronized tuning control due to the fact that the received high frequency oscillations are amplitude modulated. As a result, the tuning control potential fluctuates according to the modulating frequency, whereby for very low instantaneous amplitudes of the received oscillations, the tuning regulation may become temporarily inplitude control serving signals is shown inFigure 10 which otherwise corresponds to Figure '7. In the latter, the control potential varying in accordance with the amplitude or signal strength is derived from the discriminator circuit 34 or from a special circuit arrangement provided for this purpose. When using a discriminator of the type according to Figure 5, this amplitude responsive potential '01 may be derived from points q-r as described previously. After the higher disturbing frequencies have been eliminated by a filter M, the control potential m is used to control the gain of the amplifier 33. In general, the amplitude or volume control arrangement may be similar to the systems extensively used in radio circuits with the exception that the time constants have to be designed to be substantially lower than the time constants used in the conventional circuits for comparatively' slow variations, i. e. of the order of M and less of the time constants required in the conventional AVC. systems. Such a fast acting volume control may be combined with a normal volume control arrangement such as shown in Figure 11. In the latter, the filter 4| is designed in such a manner as to be permeable for the higherfrequencies m of the volume control potential 01 serving to instantaneously control the gain of the amplifier 33 and to limit the low frequency amplitude 'modulation at the output of the amplifier to a minimum. The mean or average value m of the AVG potential 121 on the other hand is transmitted through a filter 43 having a time constant corresponding to the conventional AVC circuits and applied to a gain control element of an RF amplifier 42 in a known manner. In this case, the intermediate frequency e2 cannot be employed after amplification in 33 for deriving thelow frequency signals due to the fact that the low frequency amplitude modulation is compensated in this amplifier 33 by the potential in. The limiting of the amplitude e: to a constant value may also be effected by means of elements having an operating characteristic with a sharp bend such as a saturation characteristic.

The principal circuits according to Figures 6,- '7, 10 and 11 merely constitute'examples for' carrying out the invention whereby all unnecessary auxiliary devices have been omitted as being su- -perfiuous for an understanding of the invention. I

The novel fast acting tuning control is specially suited for use in receiving circuits for ultra short waves employing multiple heterodyning whereby the tuning control may be applied to one or several of the auxiliary oscillators. A possible arrangement ofthis type is illustrated in Figure 12. In the latter, the incoming high frequency signal eo is impressed upon a first mixer 45 and combined with an auxiliary frequency e4 produced by a first oscillator 49. The intermediate frequency an is impressed upon an amplifier 46 having a wide response or band width such as an aperiodic amplifier, wherebythe sig-' nals are amplified to a suflicient level for operating the discriminator 41. The tuning control potential ul produced by the latter is passed through a smoothing device or filter 48 and serves to control the frequency of the local oscillations generated by the oscillator 49 in a manner described hereinbefore. The amplifier 4.6 has to be designed with a sufliciently wide band width; due to the large and rapid variations of the received high frequency oscillations and therefore a low time constant of this amplifier is required. The fluctuations of the first intermediate frequency'are considerably reduced due to this arrangement so that the first IF. amplifier 56 may be designed with a correspondingly smaller band width which however still exceeds the frequency range determined by the amplitude modulation of the received oscillations. As a result, disturbing frequencies which are at a great distance from the received instantaneous frequency will produce intermediate frequencies which are transmitted through the amplifier 46 and will result in a corresponding error in the synchronized tuning control of the first oscillator. This tuning error however is equalized by a similar; synchronized control of the second local oscillator 55. The disturbing frequencies are without.

importance and are eliminated to a substantial extent by the comparatively narrow frequency range or band width of the IF amplifier 50. The remaining frequency fluctuations especially those produced by interfering signals in the first intermediate frequency section are eliminated to a substantial extent after additional superheterodyning of esin a second mixer 5| with the local frequency e1 produced by the oscillator 55. The

second intermediate frequency signal 66 is amplified by an amplifier 52 and applied to a tune discriminator 53 to produce a tuning control potential us smoothened by a'filter 54 (ur) and applied to the oscillator 55 in a manner substantially similar as previously described. In this manner frequency fluctuations of the second intermediate frequency are reduced to aminimum and as a result thereof the amplifier 56 may be designed with a relatively narrow band width thus substantially eliminating the remaining dis- "turbing frequencies in the final oscillations (ea).- If desired, heterodyning to a further still lower frequency may be employed before final demodulation of the signals to further improve the results obtained by the invention. I

Referring to Figure 13, there is shown a complete circuit diagram for a superheterodyne receiver embodying the improvements according to the present invention and especially suited for short and ultra short waves, although not limited thereto.

Modulated high frequency signals such as am- .plitude modulated signals which are frequency modulated in accordance with the low frequency or modulating signal wave to be transmitted in addition to amplitude modulation and which may be further subject to relatively slow carrier frequency fluctuations due to fading and other causes, are intercepted by any antenna 60 of any suitable design and impressed through a tuned input transformer 6| upon the grid of an.

trol grids of an electronic mixer or frequency =15 converter valve 64.01 known type forming a frequency changer stage designated M in the drawing. The oscillating signal potential 04 of different frequency is supplied by a local oscillator O and impressed upon a separate control grid of the valve 64, preferably separated from the first control grid by a positively biased screen in the manner well known whereby intermediate or beat frequency signals e; are obtained in the secondary of a tuned coupling transformer 65 inserted in the anode or output circuit of the valve 64. The intermediate frequency signals e1 are amplified in a known manner by means of an intermediate frequency transformer IF comprising in the example illustrated a. pair of electron valve amplifiers 66 and 68 coupled in cascade through an interstage trahsformer 6! fixedly tuned to the intermediate frequency. The amplified intermediate frequency signals ea are applied to a detector circuit or demodulator such as a double diode detector 10 in the example shown through a further tuned transformer 69 The demodulated or low frequency signal currents which may be audio, video or any other signal current variations are further amplified by means of an audio frequency amplifier AF comprising in the example shown an input or driver stage 1| coupled to a push-pull output stage 'l3-l4 through an audio frequency transformer 12. The amplified audio signals supplied from the secondary terminals 18 of the push-pull output transformer 15 may serveto operate a suitable translating device such as a loud speaker in a manner well understood.

So far the system corresponds to the conventional superheterodyne receiver now being in general use. There is further provided in accordance with the present invention an arrangement to eliminate the effect of periodic relatively rapid variations of the carrier fre-.

quency suchas caused by a frequency modulation in addition to an existing amplitude modu- I lation according to a low frequency or signalling wave being transmitted, as well as of relatively slow more or less irregular fluctuations of the carrier frequency due to fading, temperature influence and other causes. For this purpose a portion of the intermediate frequency signal energy produced by the mixer M and derived from a separate tuned secondary winding of the transformer 65 is applied to an auxiliary intermediate frequency amplifier A comprising a pair of electron valves 11 and 19 coupled in cascade by a tuned interstage transformer 18. The output potential (22 of the amp ifier A is utilized to generate a tuning control potential 1L1 by means of a discriminating circuit arrangement D of sub stantially the same type as shown in Figure 5. The latter comprises a phase rotating transformer 88 and a pair of diode rectifiers BI and 82 arranged in such a manner that the vecto-rial sum and difference of the primary and secondary potentials of the transformer appears at the diodes 8| and 82, respectively. in a manner described hereinbefore. Thus, the different al 111 between the rectified potentials occurring at the opposite terminals of the condenser 83 is a measure for the detuning of the transformer 80 relative to the intermediate signal frequency of the receiver. This detuning is maintained at a sufliciently small limit by an automatic adjustment of the local oscillator frequency ed, by the aid of a control or tuning corrector device TC. For "this purpose it is necessary that rapid variations of the control potentials in are not suppressed or weakened to an appreciable extent by an excessive smoothing or filtering action. Thehigh frequency components of the control potential u: are suppressed by the by-passing condenser 83. The periodic low frequency fluctuations of the control potential are transmitted through the series condenser 84 of a filter F1 corresponding substantially to the arrangement shown and described inconnection with Figure 8. A by-passing of the low frequency fluctuations through'the condenser 85 is avoided by the provision of resistance. The time constant of the combination 84-86 is chosen sufficiently high so that the lowest audio frequency components according to which the received oscillationsare frequency modulated are transmitted without substantial attenuation. Accordingly this time constant should be of the order of from A to A0 of a second. In order to restore the controlling action automatically in case of a sudden disturbance or interruption the further condenser 85 is provided being charged to the average value u1 through resistances 81 and 86 as described in detail hereinabove. Since the average value of this control potential changes only slightly in accordance with the relatively slow average variations of the received high frequency oscillations such as due to fading, temperature or other influences the time constant of condenser 85 in conjunction with resistances 81 and 86 may be chosen of very high value such as of the order of 10 seconds. In this manner the charge of condenser 85 is not varied by instantaneous disturbances whereby the average control potential uz corresponds to the average amount of detuning, thereby maintaining the automatic tuning synchronizing action at or about the average value of the received signal frequency. -In other words, the filter F1 acts to suppress the inaudible low components of the control potential 1n; the received signal oscillations usually not being frequency modulated in accordance with these inaudible low frequencies. On the other hand, the filter transmits the direct current component serving for effecting automatic tuning upon the average value of the re ceived signal frequency as well as the audible low frequency components serving for effecting rapid automatic or synchronized tuning in the rhythm of a frequency modulation of the received signal oscillations. l

The tuning adjustment of the oscillator 0 may be effected in any suitable manner such as by the employment of an electron valve representing a variable reactance regulated by the tune control potential 11.2 as a biasing potential.

In the example illustrated the oscillator valve 88 has an oscillatory or tank circuit 89 connected between its grid and cathode and coupled with the anode circuit to generate sustained local oscillations in a manner well known. There is further connected across the tank circuit 89 an elec tron valve 98 serving as a variable tuning reactance and a series combination comprising an plied to the'control grid of the valve 90 through a series resistance 93.

According to a further feature there is derived from the discriminating circuit D a control potential or varying according to the amplitude of the intermediate frequency signals. In the example illustrated this potential is derived from the output of a single rectifier (82) of the discriminator system, while the frequency or tuning control potential 111 is derived from the outputs of both rectifiers BI and 82 in the manner described hereinbefore. The amplitude responsive potential In is freed from high frequency components by a filter F2 designed to pass the audible or low frequencies. The thus obtained output potential w of the filter F2 which varies in accordance with the low or modulating frequency signal wave is impressed through the secondary of the coupling transformer or band-pass filter 65 upon the input grid of the auxiliary intermediate frequency amplifier A, whereby the amplitude variations or modulation of the amplified intermediate frequency potentials e2 impressed upon the discriminator D are substantially suppressed, and the rapid automatic frequency control or synchronized tuning is independent of the degree of amplitude modulation of the signals received. The amplitude responsive potential 021 is applied to a further filter F3 to suppress the audible or low frequencies thereby obtaining a control potential 113 at the output of the filter Fa varying in accordance with relatively slow fiuctuations of the carrier amplitude of the received signals such as caused by fading, effects of temperature, drift, etc. This potential us which corresponds to the conventional arrangements for automatic volume control (AVC) is applied to the grid of the RF amplifier 62 to effect a normal automatic volume controL' In accordance with the different functions of the potentials v2 and v: the time constant of the filter F2 should be of the order of A to ,6 of a second and the time constant of F: of the order of from one to about of a second.

The method hereinabove described for effecting synchronized tuning is. well suited for receivlng an amplitude modulated high frequency wave subject .to additional frequency and/or phase modulation in accordance with variations of a low frequency or signal wave to be transmitted or of other disturbing frequencies, such as fluctuations of the operating potentials and the like. 1

chronized tuning, the variations of the intermediate frequency signals e1 are reduced to a minimum. Therefore, the transformers or band filters of the intermediate frequency amplifier IF may be designed with a corresponding narrow band width. On the other hand, it is obvious that the band filter of the pre-amplifier RF must be designed to encompass the entire range of frequency fluctuation and modulation band width of the received signal wave. The latter can be easily accomplished in practice due to the fact that a greater band width can be easily obtained in case of high frequencies with normal coupling arrangements'and damping values. As has been pointed out, the transmitting range of the transformers or band filters in the auxiliary intermediate amplifier A should be designed in such a manner as to avoid undesired building up of oscillations or phase rotation dependent on frequency and liable to impair the function of the rapid or synchronized tuning regulation of the entire receiver.

It will be evident from the above that the invention is not limited to the specific exemplifications and circuits as well as methods disclosed herein for illustration, but that thenovel concept and underlying principle disclosed are subject to numerous variations and modifications coming within the broader scope and spirit of the invention as defined in the appended claims.

The specification and drawings are accordingly to be regarded in an illustrative rather than in a limiting sense.

I claim:

1. A receiver system adapted to receive a high frequency carrier having its amplitudes modulated at signal frequencies within a predetermined frequency range, said carrier fluctuating in frequency at the rate of said signal frequencies, comprising a high frequency receiver channel tuned to a predetermined frequency and having a band pass of a width substantially that of said signal frequency range, frequency discriminating means responsive to said fluctuations and control means energized by said discriminating means to convert said fluctuating carrier to a second carrier the frequency of which is substantially constant and substantially equal to the predetermined frequency of said receiver channel, the time constant of said discriminating and control means being at the most of the order of of a second, whereby said carrier fluctuations are substantially instantaneously compensated.

2. A receiver system adapted to receive a high frequency carrier having its amplitudes modulated at signal frequencies within a predetermined frequency range, said carrier fluctuating at the rate of said signal frequencies, comprising an input channel having a frequency response range substantially broader than and embracing said signal frequency range and two outputs, frequency discriminating means responsive to said fluctuations and control means energized by said discriminating means to convert said fluctuating carrier into a second carrier the frequency of which is substantially constant, said discriminating means associated with one of said outputs, the time constant of said discriminating and control means being at the most of the order of of a second, whereby said carrier fluctuations are substantially instantaneously compensated, and a. high frequency amplifying channel associated with the other of said outputs and substantially tuned to said second carrier frequency, said amplifying channel having a band pass of a width substantially that of said signal frequency range. I

3. A superheterodyne receiver system adapted to receive a high frequency carrier having its amplitudes modulated at signal frequency within a predetermined frequency range, said carrier fluctuating in frequency at the rate of said signal frequencies, comprising an input channel having a frequency response range substantially broader than and embracing said signal frequencyrange and including a mixer having two outputs, a

source of local oscillations associated with said mixer so as to produce a second carrier of intermediate frequency in said mixer, frequency discriminating means responsive to said fluctuations and associated with an output of said mixer so as to derive carrier energy of intermediate frequency therefrom, said discriminating means adapted to develop voltages representative of magnitude and sense of said fluctuations, the

10 time constant of said discriminating means ,being at the most of the order of $1000 of a second so that said voltages are developed substantially instantaneously, control means energized by said discriminating means for applying said voltages to said source of local oscillations so as to cause instantaneous fluctuations of the latter in such sense that a substantially constant intermediate frequency results in said mixer, and a high frequency channel fixedly tuned to said constant .0 intermediate frequency and having a band pass width substantially that of said signal frequency range, said latter channel associated with another output of said mixer.

4. In a receiver system as described in claim 85 3, means associated with said discriminating means for developing a voltage representative of the amplitude 'of said second carrier, amplifying means associated with and preceding said discriminating means, said amplifying means ino cluding a gain control element, and means for applying said voltage to said control element in a manner so as to counteract variations in said amplitude. I

5. In a receiver system as described in claim 2, a means associated with said discriminating means for developinga potential representative of the amplitude of said fluctuating carrier, and means responsive to said potential to counteract amplitude variations in the carrier applied to said 0 discriminating means.

6. In a receiver as claimed in claim 1, current limiting means for substantially suppressing amplitude variations in the carrier applied to said discriminating means.

5 7. A receiver system adapted to receive a high frequency carrier having its amplitudes modulated at signal frequencies within a predetermined frequency range, said carrier fluctuating in frequency rapidly at 'the rate of said signal frequencies and slowly below said rate, comprising a high frequency receiver channel tuned to a predetermined frequency and having a band pass of a width substantially that of said signal frequency 5 range, frequency discriminating means responsive to all said fluctuations and control means energized by said discriminating means to convert said fluctuating carrier into a second carrier the frequency of which is substantially constant and 10' substantially equal to the predetermined frequency of said receiver channel, the output of said discriminating means comprising a com-' pound filter network, one part of said network having a time constant at the most of the order 18 of 5 of a second, whereby said rapid fluctuations of said fluctuating carrier are substantially instantaneously compensated, another part of said network having a considerably larger time constant, whereby said slow fluctuations of said 50 fluct ating carrier are substantially compensated.

8. A receiver system adapted to receive a high frequency carrier having its amplitudes modulated at signal frequencies within a predetermined frequency range, said carrier fluctuating in frequency rapidly at the rate of said signal frequencies and slowly below said rate-comprising a high frequency receiver channel tuned to a predetermined frequency and having a band pass of a width substantially that of said signal fre- 80 quency range, frequency discriminating means comprising means responsive to said rapid fluctuations and means responsive to said slow fluetuations, said rapidly and slowly responsive means arranged to convert said fluctuating carrier into a second carrier the frequency of which is substantially constant and substantially equal to the predetermined frequency of said receiver channel, the time constant of said rapidly responsive means being at the most of the order of 40 ,5 of a second, whereby said rapid fluctuations of said fluctuating carrier are substantially instantaneously compensated, said slowly responsive means having a considerably larger time constant, whereby said slow fluctuations of said fluctuating carrier are substantially compensated.

GUSTAVE GUANELLA. 

